1. Field of the Invention
The present invention relates to an image-forcing device, particularly, to a scanning unit installed in the image forming device for deflecting a light beam and scanning a surface of a photosensitive member with the deflected light beam.
2. Description of the Related Art
In a conventional electrophotographic image-forming device such as a laser printer, a light-emitting unit provided with a laser diode irradiates a laser beam onto the surface of a photosensitive member carrying a uniform electrical charge, forming an electrostatic latent image thereon. The latent image is developed into a visible image with toner, and the resulting toner image is transferred onto a sheet of paper or other recording medium to form images thereon.
Generally, this type of image-forming device scans the laser beam over the surface of the photosensitive member by deflecting the laser beam with a polygon mirror rotating at a high rate of speed. In order to control the timing at which the beam emitted from the light-emitting unit starts irradiating at an appropriate position on the surface of the photosensitive member, a beam detect sensor (BD sensor) is commonly used to detect when the deflected light beam has passed a prescribed position outside the photosensitive member. When a prescribed time has elapsed after the BD sensor detects the light beam, the deflected light beam starts scanning on the photosensitive member being modulated with image data to form an electrostatic latent image at an appropriate position on the surface of the photosensitive member.
However, conventional BD sensors are less sensitive than the photosensitive members. In addition, the sensing level of the BD sensors to detect the light beam having a light intensity for exposing the photosensitive member is not stable. For these reasons, Japanese unexamined patent application publications 11-218697 and 2002-6246 disclose an image exposure device eliminating the usage of the BD sensor.
The image exposure device disclosed in Japanese unexamined patent application publication 2002-6246 includes a transparent disc provided integrally and concentrically with the polygon mirror, a reference mark formed of an opaque material disposed at a prescribed location on the transparent disc, a photointerrupter for detecting the reference mark, and a pulse generator for generating pulses. A start signal is generated at a time t1 in synchronism with the first pulse generated by the pulse generator after the reference mark has been detected, The scanning process begins at a time t2 after a prescribed time duration ΔT after the time t1.
Further, manufacturing errors in the polygon mirror cannot be ignored when deflecting a light beam with the mirror. For example, as disclosed in Japanese unexamined patent application publication 2002-6246, when the time duration ΔT again elapses after the time t4, which comes after a prescribed number of drive pulses from the time t1 of the start signal, manufacturing errors in the polygon mirror can cause the exposure position to become slightly offset with each scanning line. The conventional technologies described above do not account for manufacturing error of each flat reflecting surface of the polygon mirror which is inherent in individual polygon mirrors.
In view of the foregoing, it is an object of the present invention to provide an image-forming device that precisely controls scanning a photosensitive member with a light beam modulated by image data to be formed.